Let me report I reinstalled again, had a 20-minute FSX session and everything worked just fine. I don't know how you did it but it felt like childhood's memories of magicians.
I have no idea why it works now and it didn't worked before. I start to suspect that, the first time we update a file, some antivirus will block it by default, just because it's "new = unknown to them".
Just to confirm how defective antivirus can be these days: I realized my FSUIPC was one version behind, so I downloaded it again from the official site and, guess what, Windows defender REMOVED THE INSTALLER!! I had to exclude the folder where I downloaded, otherwise the antivirus won't even let me unzip it! It said Install_FSUIPC4.exe is a Trojan...